Selective catalytic reduction of NOx enabled by sidestream urea decomposition

ABSTRACT

A preferred apparatus arrangement utilizes the enthalpy of the flue gas, which can be supplemented if need be, to convert urea ( 30 ) into ammonia for SCR. Urea ( 30 ), which decomposes at temperatures above 140 .degree. C., is injected ( 32 ) into a flue gas stream split off ( 28 ) after a heat exchanger ( 22 ), such as a primary superheater or an economizer. Ideally, the side stream would gasify the urea without need for further heating; but, when heat is required it is far less than would be needed to heat either the entire effluent ( 23 ) or the urea ( 30 ). This side stream, typically less than 3% of the flue gas, provides the required temperature and residence time for complete decomposition of urea ( 30 ). A cyclonic separator can be used to remove particulates and completely mix the reagent and flue gas. This stream can then be directed to an injection grid ( 37 ) ahead of SCR using a blower ( 36 ). The mixing with the flue gas is facilitated due to an order of magnitude higher mass of side stream compared to that injected through the AIG in a traditional ammonia-SCR process.

BACKGROUND OF THE INVENTION

The invention concerns a new selective catalytic reduction of NO_(x),which is enabled by a side gas stream that can be separated from themain stream, or a side stream of hot air to provide for decomposition ofthe urea into its active components including ammonia.

Efforts are being made in many jurisdictions to reduce the emissions ofnitrogen oxides (NO_(x)). The technologies have included those thatmodify the combustion conditions and fuels, known as primary measures,and those that treat the exhaust after combustion, known as secondarymeasures. When effective primary measures are employed, the secondarymeasures can still be employed to achieve further reductions. To providethe best NO_(x) reduction, it is apparent that both primary andsecondary measures will be necessary.

Among the known secondary measures are selective catalytic reduction(SCR) and selective noncatalytic reduction (SNCR). Both have beenconducted with both ammonia and urea. See, for example U.S. Pat. No.3,900,554, wherein Lyon discloses SNCR of nitrogen monoxide (NO) in acombustion effluent by injecting ammonia, specified ammonia precursorsor their aqueous solutions into the effluent for mixing with thenitrogen monoxide at a temperature within the range of 1600° F. to 2000°F. Lyon also suggests the use of reducing agents, such as hydrogen orvarious hydrocarbons, to permit the effective use of ammonia at effluenttemperatures as low as 1300° F. However, these temperatures are oftentoo high for effective treatment, ammonia is difficult to deal withsafely, and SNCR is not as effective as SCR. Similar processes aretaught for urea by Arand, Muzio, and Sotter, in U.S. Pat. No. 4,208,386,and Arand, Muzio, and Teixeira, in U.S. Pat. No. 4,325,924. Again thetemperatures are high, and the use of lower temperatures is not enabled.

SCR can operate with ammonia at lower temperatures, generally within therange of from 100° to 900° F. In this regard, see U.S. Pat. Nos.3,032,387 and 3,599,427. SCR (selective catalytic reduction) has beenavailable for years in some contexts for reducing NO_(x). To date,however, SCR has depended on the use of ammonia, which has safetyproblems associated with its storage, handling, and transport. Urea issafer, but has not been practical for many SCR applications due to thedifficulty in converting it from a solid or an aqueous form to itsactive gaseous species that are reactive on catalyst bed for NOxreduction. Also, the reagent economics typically favor anhydrous ammoniaover urea. In “A Selective Catalytic Reduction Of NO_(x) from DieselEngines Using Injection Of Urea” (Ph.D. thesis, September 1995),Hultermans describes a number of technical challenges in the context ofDiesel engines while giving a broad background on the technology.

The use of catalysts for NO_(x) reduction utilizing urea is effectivebut is sensitive to particulates and undecomposed urea, which can foul acatalyst. In this regard, it must be remembered that temperatures at thelow end of the SCR treatment temperature range will not be high enoughto fully gasify the urea. In addition, SCR requires very uniform mixingof active gaseous species prior to contact with the catalyst, and it isdifficult to uniformly mix urea or its decomposition products with thelarge amounts of effluent in need of treatment. The limited attempts touse urea SCR for stationary and mobile sources, such as diesel engines,have been described in several recent patents including U.S. Pat. No.5,431,893, to Hug, et al. To protect the catalyst from fouling, Hug, etal., proposes bulky equipment capable of treating all effluent withurea. Regardless of physical form, urea takes time to break down in hotexhaust gases and may cause nozzle plugging at the temperatures mostconducive to gasification. This disclosure highlights the problemsmaking it a necessity that the urea solution is maintained at atemperature below 100 C to prevent hydrolysis in the injectionequipment. They propose the use of moderate urea pressures when feedingthe urea and find it necessary to have alternative means to introducehigh-pressure air into the feed line when it becomes plugged. Thenozzles employed by Hug, et al., use auxiliary air to aid dispersion.Also, they employ dilute solutions that require significant heating tosimply evaporate the water. See also, WO 97/01387 and European PatentSpecification 487,886 A1.

In European Patent Specification 615,777 A1, there is described anapparatus that feeds solid urea into a channel containing exhaust gases,which are said to be hydrolyzed in the presence of a catalyst. Forsuccessful operation the disclosure indicates that it is necessary toemploy a hydrolysis catalyst, compressed air for dispersion of finesolids, means for grinding the urea into fine solids and a coating toprevent urea prills from sticking together. The disclosure notes that ifthe inside of the catalyzer and the nozzle tip only were coated with thecatalyst, corrosion and deposition would occur. Despite achieving thegoal of removing water from the process, the specification introducessolid urea into the gas stream—possibly depositing urea on the SCRcatalyst.

In U.S. Pat. No. 6,146,605 to Spokoyny, there is described a combinedSCR/SNCR process in a staged process involving a separate step ofhydrolyzing the urea prior to an SCR stage. A similar process isdisclosed in U.S. Pat. Nos. 5,985,224 and 6,093,380 to Lagana, et al.,which describe a method and apparatus involving the hydrolysis of ureafollowed by a separation of a gas phase from a liquid hydrolysate phase.Also, Copper, et al., disclosed a urea hydrolysis process to generateammonia in U.S. Pat. No. 6,077,491. In all these processes there is arequirement to handle a significant amount of high temperature and highpressure gas and liquid phases containing ammonia during and afterhydrolysis. This extra processing requires the purchase and maintenanceof additional equipment, an emergercy plan and equipment to handleammonia release in case of process failures, and it would be desirableto have a system which operated more safely, simply and efficiently.

The art is awaiting the development of a process and apparatus thatwould permit the use of urea in an SCR process simply, reliably,economically, and safely.

SUMMARY OF THE INVENTION

The invention provides a practical way to achieve uniform mixing ofactive gaseous reactants for NO_(x) reduction by SCR using urea as thereagent and novel process arrangements that assure that the gases are atthe proper temperature for effective NO_(x) reduction.

The new design of the invention enables gasification of urea andthorough mixing with NO_(x)-containing combustion gases and canadvantageously utilize the enthalpy of the flue gas, which can besupplemented if need be, to convert urea to SCR reagents such asammonia. Urea, which decomposes at temperatures above 140° C., isinjected into a side stream where it is gasified and mixed with othergases. In one highly effective arrangement, the side stream is a fluegas stream split off after a primary superheater or an economizer.Ideally, the side stream would decompose the urea without need forfurther heating; but, when heat is required, it is far less than wouldbe needed to heat either the entire effluent. Depending on thetemperature, this side stream, typically less than 3% of the flue gas,provides the required enthalpy and momentum for complete gasification ofurea and thorough mixing of the reagent containing side stream into themain stream.

A mixing device, such as cyclonic separator, static mixer or blower, canmore completely mix the reagent and flue gas prior to reinjection intothe main stream. A cyclone separator has the advantage that it can alsoremove particulates that might be present. The side stream containinggasified urea can then be directed to an injection grid ahead of an SCRcatalyst using a high temperature blower. Vortex mixers or other typesof static mixer can be installed downstream of the injection grid tothoroughly mix the side stream and the main stream. The mixing with theflue gas is facilitated due to an order of magnitude higher mass of sidestream compared to that injected through the ammonia-injection grid(AIG) in a traditional ammonia-SCR process.

This new process and the apparatus for performing it make use of theeasy handling feature of urea reagent without requiring either reagentcarrier air or an additional source of heat solely directed to heat andhydrolyze the urea, and the side stream gas mass provides thoroughmixing required for high levels of NO_(x) reduction.

According to one embodiment of the invention, a side stream is separatedfrom the main effluent stream from a combustor and urea is injected intoit at a temperature sufficient to fully decompose or otherwise gasifythe urea to active gas species.

According to another embodiment of the invention, a side stream isseparated from the main effluent stream from a combustor following finaltreatment and urea is injected into it at a temperature sufficient tofully gasify the urea to active gas species.

According to a further embodiment of the invention, a side stream isbrought in from a source external to the combustor and urea is injectedinto it at a temperature sufficient to fully gasify the urea to activegas species.

According to another embodiment of the invention, a side stream isseparated from the main effluent stream from a combustor, a heater isprovided to raise the split stream temperature sufficiently to fullygasify the urea to active gas species and urea is injected into itwherein it is decomposed or otherwise gasified.

According to another embodiment of the invention, a side stream isseparated from the main effluent stream from a combustor and urea isinjected into it, with the two streams then combined and passed througha cyclone to effect complete mixing and particle separation.

In another embodiment of the invention, a side stream is separated fromthe main effluent stream from a combustor and is passed through acyclone prior to heating it and injecting urea into it.

In yet another embodiment of the invention, a side stream is separatedfrom the main effluent stream from a combustor, and the stream is heatedand urea is injected into it prior to passing it through a cyclone.

An alternative embodiment of the invention, utilizes a stream of air,air preheated by a flue-gas-to-air heat exchanger, or preheatedcombustion air, which is further heated and combined with urea, with theresulting stream then passed through a mixer, if desired, and injectiongrid as it is combined with the effluent stream from a combustor andpassed through an SCR reactor.

In any of these embodiments, steam can be employed to assure maximumproduction of ammonia and as supplemental source of heat forgasification or for maintaining the temperature of the catalyst. Also,the side stream containing active SCR reagent can be reintroduced intothe main flue gas stream through a properly designed ammonia injectiongrid (AIG) in a traditional ammonia-SCR process in any of theseembodiments. Furthermore, a blower appropriate for supplying air or fluegas at desired temperatures can be located before or after the ureainjection, whichever better suited for an application, to providesufficient pressure to reintroduce the side stream into the main fluegas stream.

Many of the preferred aspects of the invention are described below.Equivalent compositions are contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its advantages will becomemore apparent from the following detailed description, especially whentaken with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of one embodiment of the inventionwherein a side stream is separated from the main effluent stream from acombustor and urea is injected into it at a temperature sufficient tofully decompose the urea to active gas species. A blower that can belocated before or after the urea injection provides sufficient pressureto introduce the side stream into the main stream. An injection grid ora traditional ammonia injection grid is utilized to thoroughlydistribute the reagent into the main stream ahead of SCR reactor.

FIG. 2 is a schematic representation of another embodiment of theinvention wherein a side stream is separated from the main effluentstream from a combustor and a burner or other means of heating the fluegas is provided to raise the split stream temperature sufficiently tofully decompose the urea to active gas species and urea is injected intoit wherein it is decomposed or otherwise gasified.

FIG. 3 is a schematic representation of another embodiment of theinvention wherein a side stream is separated from the main effluentstream from a combustor and urea is injected into it, and is optionallyheated, with the two streams then combined and passed through a cycloneto effect complete mixing and particle separation.

FIG. 4 is a schematic representation of another embodiment of theinvention wherein a side stream is separated from the main effluentstream from a combustor and is passed through a cyclone prior to heatingand injecting urea into it, with the side stream advanced through aninjection grid with the aid of a blower.

FIG. 5 is a schematic representation of another embodiment of theinvention wherein a side stream is separated from the main effluentstream from a combustor and the stream is heated and injected with ureaprior to passing it through a cyclone.

FIG. 6 is a schematic representation of another embodiment of theinvention wherein a stream of air is heated and urea is injected intoit, with the resulting stream being then passed through a mixer, ifdesirable, and injection grid and combined with the effluent stream froma combustor and passed through an SCR reactor.

FIG. 7 is a schematic representation of another embodiment of theinvention similar to FIG. 6, wherein steam is employed as the heatsource.

FIG. 8 is a schematic representation of another embodiment of theinvention similar to FIG. 7, wherein the steam is introduced followingintroduction of the urea.

FIG. 9 is a schematic representation of another embodiment of theinvention wherein the side stream is formed from combustion gasesfollowing treatment in the SCR reactor, with the resulting stream beingheated and injected with urea.

FIG. 10 is a schematic representation of another embodiment of theinvention wherein the side stream is formed from combustion gasesfollowing treatment in the SCR reactor and a particulate collectiondevice such as ESP, bagfilter, or cyclonic separators, with theresulting stream being heated and injected with urea.

FIG. 11 is a modification of urea injection from FIG. 6, wherein insteadof aqueous urea injection, a finely ground, pulverized or micronizedsolid urea is injected using pneumatic carrier air.

DESCRIPTION OF THE INVENTION

The invention provides a urea-based SCR process that can advantageouslyutilize the enthalpy of the flue gas, which can be supplemented if needbe, to convert urea to ammonia. There are several embodiments which willbe described in preferred forms. It is intended, however, that variousfeatures of the embodiments can be utilized in combination withembodiments other than those specifically detailing the features. Commonelements and features of the drawings will have common referencenumerals throughout the drawings.

This new process makes use of the easy handling feature of urea reagentand provides complete gasification and good mixing employing a sidestream gas mass to provide thorough mixing required for high levels ofNO_(x) reduction. In particularly advantageous embodiments, heatnecessary for gasification is derived from the enthalpy of thecombustion gases.

The process is effective with urea, but can utilize otherNO_(x)-reducing reagents capable of generating a reactant gas containingammonia upon heating. As will be clear from the following, when certainof these reagents are gasified, the reactant gas will also contain HNCOwhich reacts with water to convert to ammonia and carbon dioxide. It isan advantage of the invention that this can be easily achieved withoutprehydrolysis of the NO_(x)-reducing reagent which has the attendantrisk of plugging nozzles and other equipment. By the term “gasification”we mean that substantially all of the urea is converted into a gas,leaving no significant dissolved or free solids or liquid to contactwith and foul SCR catalysts.

The term “urea” is meant to include the reagents that are equivalent tourea in the sense that they form ammonia and HNCO when heated, whetheror not they contain large amounts of the pure chemical urea in the formintroduced into the combustion gases; however, the reagents that areequivalent to urea typically contain measurable quantities of urea intheir commercial forms and thus comprise urea. Among the NO_(x)-reducingreagents that can be gasified are those that comprise a member selectedfrom the group consisting of: ammelide; ammeline; ammonium carbonate;ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammoniumsalts of inorganic acids, including sulfuric acid and phosphoric acid;ammonium salts of organic acids, including formic and acetic acid;biuret; triuret, cyanuric acid; isocyanic acid; urea formaldehyde;melamine; tricyanourea and mixtures of any number of these. Yet otherNO_(x)-reducing reagents are available that do not form HNCO, butdecompose to a mixture of gases including hydrocarbons. Among this groupare various amines and their salts (especially their carbonates),including guanidine, guanidine carbonate, methyl amine carbonate, ethylamine carbonate, dimethyl amine carbonate, hexamethylamine;hexamethylamine carbonate; and byproduct wastes containing urea from achemical process. Amines with higher alkyls can be employed to theextent that the hydrocarbon components released do not interfere withthe NO_(x)-reduction reaction.

The term “urea” is thus meant to encompass urea in all of its commercialand equivalent forms. Typically, commercial forms of urea will consistessentially of urea, containing 95% or more urea by weight. Thisrelatively pure form of urea is preferred and has several advantages inthe process of the invention.

It is a further advantage of the invention, that while ammonia need notbe utilized at all, the apparatus described herein improves theintroduction of SCR reagents including ammonia and thus makes its usemore practical even though the problems with its storage will not befully resolved.

The urea solution is introduced at a rate relative to the NO_(x)concentration in said combined stream prior to passage through saidNO_(x)-reducing catalyst effective to provide an NSR of from about 0.1to about 2, depending on a number of factors, but more typically iswithin the range of from 0.5 to 1.1. The term “NSR” refers to therelative equivalents of nitrogen in the urea or other NO_(x)-reducingagent to the equivalents of nitrogen in the NO_(x) in the combustiongases to be treated.

The term “combustor” is meant in the broad sense to include allcombustors which combust carbonaceous fuels to provide heat, e.g., fordirect or indirect conversion to mechanical or electrical energy. Thesecarbonaceous fuels can include the hydrocarbons normally used as fuelsas well as combustible waste materials such as municipal solid waste,industrial process waste and the like. Burners and furnaces, as well as,internal combustion engines of the Otto, Diesel and turbine types, areincluded within the definition of the term combustor and can benefitfrom the invention. However, since the problems and advantages ofsuccessful achievement of reliable NO_(x) reduction on combustorsutilizing ammonia as a reducing agent are so pronounced, the large-scalecombustor is used throughout this description for purposes of example.Stationary and mobile combustors of all types are, however,contemplated. However, the current invention is not limited to combustorflue gases. Rather, any hot flue gas that can benefit from passingthrough an SCR reactor for NOx reduction can benefit from thisinvention.

FIG. 1 is a schematic representation of one embodiment of the inventionwherein a side stream is separated from the main effluent stream from acombustor and urea is injected into it at a temperature sufficient tofully decompose or otherwise gasify the urea to active gas species. Alarge-scale combustor 20 burns fuel with the resulting production ofnitrogen oxides (NO_(x)) that must be at least partially removed. Thecombustion gases are used to heat water in heat exchanger array 22before the combustion gases are exhausted to the atmosphere by passage23 and apparatus downstream. A mixing device 24 is optional followingadding urea reagent to a side stream and combining the side stream witha main combustion gas stream as will be explained. The term “sidestream” is used herein to refer to a stream of relatively small volumerelative to the total volume of combustion gases to be treated bygasified urea and NO_(x)-reduction catalysts, 26, 26′ and 26″. The sidestream can be obtained by splitting off a side stream portion 28 of thefull stream of combustion gases in passage 23 leaving principal stream29 of combustion gases. The separation in various embodiments will bemade before or after treatment. In addition, the side stream can beformed by drawing in a stream of air from sources external of thecombustor.

Catalysts 26, 26′ and 26″ are employed in an array forming a reactor andare SCR catalysts as known in the art for reducing NO_(x) utilizingammonia or urea in various hydrolyzed, gasified, pyrolyzed and likeforms. Among the suitable SCR catalysts are those capable of reducingthe effluent nitrogen oxides concentration in the presence of ammonia.These include, for instance, activated carbon, charcoal or coke,zeolites, vanadium oxide, tungsten oxide, titanium oxide, iron oxide,copper oxide, manganese oxide, chromium oxide, noble metals such asplatinum group metals like platinum, palladium, rhodium, and iridium, ormixtures of these. Other SCR catalyst materials conventional in the artand familiar to the skilled artisan can also be utilized. These SCRcatalyst materials are typically mounted on a support such as a metal,ceramic, zeolite, or homogeneous monolith, although other art-knownsupports can also be used.

Among the useful SCR catalysts are those representative prior artprocesses described below. Selective catalytic reduction processes forreducing NO_(x) are well known and utilize a variety of catalyticagents. For instance, in European Patent Application WO 210,392,Eichholtz and Weiler discuss the catalytic removal of nitrogen oxidesusing activated charcoal or activated coke, with the addition ofammonia, as a catalyst. Kato, et al., in U.S. Pat. No. 4,138,469 andHenke in U.S. Pat. No. 4,393,031 disclose the catalytic reduction ofNO_(x) using platinum group metals and/or other metals such as titanium,copper, molybdenum, vanadium, tungsten, or oxides thereof with theaddition of ammonia to achieve the desired catalytic reduction. See alsoEP 487,886, which specifies a V₂O₅/WO₃/TiO₂ catalyst with a workingrange of 220° to 280° C. Other catalysts based on platinum can haveoperating temperatures even lower, e.g., down to about 180° C.

Another catalytic reduction process is disclosed by Canadian Patent1,100,292 to Knight, which relates to the use of a platinum group metal,gold, and/or silver catalyst deposited on a refractory oxide. Mori, etal., in U.S. Pat. No. 4,107,272, discuss the catalytic reduction ofNO_(x) using oxysulfur, sulfate, or sulfite compounds of vanadium,chromium, manganese, iron, copper, and nickel with the addition ofammonia gas.

In a multi-phased catalytic system, Ginger, in U.S. Pat. No. 4,268,488,discloses exposing a nitrogen oxides containing effluent to a firstcatalyst comprising a copper compound such as copper sulfate and asecond catalyst comprising metal combinations such as sulfates ofvanadium and iron or tungsten and iron on a carrier in the presence ofammonia.

The effluent containing the reactant gas is most preferably passed overthe SCR catalyst while the combustion gases including the gasified ureaor other reagent are at a temperature of at least about 100° C. andtypically between about 180° and about 650° C., preferably above atleast about 250° C. In this manner, the active species present in theeffluent due to gasification of the reagent solution most effectivelyfacilitate the catalytic reduction of nitrogen oxides and condensationof water is controlled. The effluent will typically contain an excess ofoxygen, e.g., up to about 15% of that required to fully oxidize thecarbonaceous fuel. Use of the present invention with any of the aboveSCR catalysts (the disclosure of which are specifically incorporated byreference) reduces or eliminates the requirement for the transport,storage and handling of large amounts of ammonia or ammonium water.

In FIG. 1, the main full stream of combustion gases in duct 23 is splitto provide side stream 28 and a principal stream 29 of volume greaterthan the side stream. Urea, which decomposes at temperatures above 140°C., is injected from storage 30 via nozzle 32 with suitable valves 34and controllers (not shown) into a flue gas stream 28 split off after aprimary superheater or an economizer (shown generally as heat exchanger22). To achieve the goal of gasification for a urea or a urea-relatedNO_(x)-reducing reagent, temperatures above about 300° C. are typicallyemployed for gasification.

The urea solution is desirably maintained at a concentration suitablefor storage and handling without precipitation or other problem.Concentrations of from about 5 to 70% can be employed with some degreeof practicality, but concentrations of from about 15 to about 50% aremore typical. It is an advantage of the invention that the amount ofwater in the urea solution can be varied alone or with steam added tosuitably control the temperature of the gases in the side stream.

The temperature of the gases produced by gasifying reagents in thisgroup should be maintained at a level that prevents their condensation.Typically, the temperature should be maintained at a temperature atleast about 150° C., and preferably at least 200° C. A preferredtemperature range for the gasification and for transfer of the gasesproduced by the noted group of reagents, is from about 300° to about650° C. Ideally, the side stream 28 would decompose the urea into activespecies without need for further heating. This side stream (e.g., from0.1 to 25% of the flue gas), typically less than 10% and usually lessthan 3%, e.g., from 0.1 to 2%, of the volume of the total combustiongases (flue gas), provides the required enthalpy for completedecomposition of urea and the sufficient momentum to mix the side stream28 with the principal stream 29 while the principal stream 29 can beutilized for further heat exchange.

The vessel carrying the side stream 28 provides the required time andgas velocity for urea decomposition. After injection, a residence timefrom 1 to 10 seconds is typically provided to completely decompose ureaand promote the reaction between HNCO and water vapor to form ammonia.Side stream gas velocity of 1 to 20 feet per second is maintainedthroughout the vessel to optimize vessel dimensions, achieve plug flow,enhance the urea droplet dispersion, evaporation, and decomposition intothe side stream, and minimize droplet impingement on vessel walls.Internal channels and multi-walls may be preferred to achieve theoptimum gas velocity and to minimize heat loss to outside environment.The optimum vessel design can be derived by using, among others,well-established design tools such as computational fluid-dynamicsmodel.

The urea injection nozzle 32 introduces well-defined droplets. Both airassisted atomizer or a mechanical atomizer can be utilized. Dropletsizes less than 500 microns but typically less than 100 and preferablybelow 50 microns are desirable to rapidly evaporate and decompose ureadroplets. Also in consideration of vessel size, small and slow dropletsgenerated from, e.g., ultrasonic nozzles can be more desirable thanlarge and fast droplets. If desired, steam can be introduced asnecessary or desired. (See FIGS. 7-9, in this regard.) This side stream28 can then be directed to an injection grid 37 (or other suitableintroduction device or apparatus such as a traditional ammonia injectiongrid) ahead of SCR reactor containing catalysts, e.g., 26, 26′ and 26″.In this embodiment, a high temperature blower 36 is employed to providea suitable injection pressure, e.g., about 1 psig or less, for theammonia injection grid 37 and additionally provides mixing.Alternatively, a high temperature blower 36 can be located upstream ofurea nozzle 32 instead of the depicted location.

A traditional ammonia injection grid 37 with densely located nozzlesrequires as low as 0.1% of the total combustor flue gas as the sidestream. A static mixer 24 can be used if desired. Alternatively,injection grid 37 can comprise fewer and sparsely-placed nozzles oropenings with a static mixer 24 located downstream to obtain a uniformdistribution. This alternate design may reduce cost and maintenanceassociated with the injection grid. The mixing with the flue gas isfacilitated due to an order of magnitude higher mass of side stream,e.g., 1 to 2% of the flue gas, compared to that injected through anammonia injection grid (AIG) in a traditional ammonia SCR process. Thus,the current embodiment provides the flexibility to the type of injectiongrid depending on the application requirements.

It is an advantage of this and other embodiments of the invention thatbecause relatively large volumes of side stream gases are mixed with theurea solution prior to introducing the gases into the SCR catalyst, anovert mixing procedure is not essential. It will be advantageous in manycases, especially where there is a high degree of fluctuation in gasvolumes, to provide means for mixing the gases at one or more stages.Among the suitable mixing means are static mixers, cyclones, blowers andother process equipment that by design or effect mixes the gases.

It is another advantage of this embodiment of the invention that byutilizing the side stream comprised of combustion gases prior to fullheat exchange, the enthalpy of the gases is utilized for gasification bydirect heat exchange with the aqueous urea solution. Surprisingly,calculations will show that direct heat exchange in this manner usingsupplementary heat only as needed under low-load conditions—when theneed for NO_(x) reduction is also low—will be much more efficient thanemploying supplementary heat in a cold stream to gasify urea.Advantageously, also, the addition of supplemental heat to the sidestream can be an effective means to control the temperature in the sidestream for consistent urea decomposition and SCR catalyst and maintainboth temperatures within its effective temperature range.

FIG. 2 illustrates an embodiment similar to that of FIG. 1, but providesheater 38 to enable increasing the temperature of the side stream 28sufficiently to assure breakdown of the urea as needed. This isespecially useful when output is low for a boiler. It is an advantage ofthis arrangement that when heat is required, the amount required is farless than would be needed to heat either the entire effluent or simplythe urea. A high temperature blower 36, located downstream of ureanozzle 32, can be located upstream of heater 38 instead of the depictedlocation. A heater 38 shown as a burner can be replaced with a steamcoil heater, heat exchanger or other means to transfer heat to the sidestream 28.

FIG. 3 is a schematic representation of another embodiment of theinvention wherein side stream 28 is separated from the main effluentstream from a combustor and heated as needed prior to injecting ureainto it. The two streams (23 and 28) are combined and passed through acyclone 40 to effect particle separation and complete mixing. A hightemperature blower 36, located downstream of urea nozzle 32, can belocated upstream of heater 38. A heater 38 can be replaced with a steamcoil heater, heat exchanger or other means to transfer heat to the sidestream 28.

FIG. 4 is a schematic representation of another embodiment of theinvention wherein side stream 28 is separated from the main effluentstream 23 from combustor 20 and is passed through a cyclone 40 (or otherparticle separating device or apparatus) prior to heating as needed byheater 38 and injecting urea into it via injector 32. A high temperatureblower 36, located downstream of urea nozzle 32, can be located upstreamof heater 38 or a cyclone separator 40. A heater 38 can be replaced witha steam coil heater, heat exchanger or other means to transfer heat tothe side stream 28.

FIG. 5 is a schematic representation of an embodiment of the inventionsimilar to that in FIG. 4, wherein side stream 28 is separated from themain effluent stream 23 from a combustor 22 is heated and urea isinjected into it just prior to or in cyclone 40 (or other particleseparating device or apparatus). The resulting treated stream is passedvia blower 36 though an injection grid 37 (or other suitableintroduction device or apparatus) ahead of the SCR reactor. Also, anoptional static mixer 39 is illustrated. A high temperature blower 36,located downstream of cyclone 40, can be located upstream of heater 38.A heater 38 can be replaced with a steam coil heater, heat exchanger orother means to transfer heat to the side stream 28.

FIG. 6 is a schematic representation of another embodiment of theinvention wherein a stream of air is forced into duct 128 and heated,and urea is injected into it via injector 32. The resulting stream isthen passed through a mixer and injection grid as it is combined withthe effluent stream from a combustor and passed through an SCR reactor.This embodiment shows heat exchanger 45 and burner 38, but either orboth can be employed as needed. Other means to transfer heat to the sidestream 28 can replace the heat exchanger 45 or a burner 38.

This embodiment is useful in situations where the configuration ofcombustor 20 does not easily permit construction of a side stream ofcombustion gases and, therefore, requires additional heat. Thisadditional heat can be lessened by using the preheated combustion aircommonly available in utility boilers.

FIG. 7 is a schematic representation of another embodiment of theinvention similar to FIG. 6, wherein steam is introduced by means 50 asthe heat source.

FIG. 8 is a schematic representation of another embodiment of theinvention similar to FIG. 7, wherein the steam source 50 is locatedfollowing introduction of the urea.

FIG. 9 is a schematic representation of another embodiment of theinvention similar to FIG. 6, wherein a side stream 228 is formed fromcombustion gases following treatment in the SCR catalyst reactor. Thisembodiment has the advantage that the gases have considerable heatvalue, especially if withdrawn prior to using them to preheat thecombustion air.

FIG. 10 is a schematic representation of another embodiment of theinvention similar to FIG. 9, wherein a side stream 328 is formed fromcombustion gases following treament in the SCR catalyst reactor anddownstream particulate collection device 60 such as an electrostaticprecipitator, bagfilter, or a cyclonic separator. While gases have lessheat value than the previous representation, this scheme offers anadvantage of being substantially particulate free when applied on solidor liquid fired combustors. Low particulates minimize maintenancerequirements.

In FIG. 11, a modification of urea injection from FIG. 6 is represented.Instead of aqueous urea injection, a finely ground, pulverized ormicronized solid urea is injected using pneumatic carrier air via line31 and nozzle 232 from line 234. This solid urea injection can beadapted into all previous representations. Without water, solid urea hasthe advantage of lower heating requirements.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible modifications and variations that will become apparent to theskilled worker upon reading the description. It is in-tended, however,that all such modifications and variations be included within the scopeof the invention that is seen in the above description and otherwisedefined by the following claims. The claims are meant to cover theindicated elements and steps in any arrangement or sequence which iseffective to meet the objectives intended for the invention, unless thecontext specifically indicates the contrary.

1. An apparatus for reducing the concentration of nitrogen oxides in astream of combustion gases, comprising: a particle separating device toremove particulates from at least a portion of the combustion gases inadvance of a NO_(x)-reducing catalyst; conduit means for transporting aside stream of gases comprised of outside air and/or combustion gasesand comprising less than 3% of the volume of the total combustion gasesat a temperature of at least 300° C.; means including nozzles to producedroplets of under 500 microns of an aqueous solution of ureaNO_(x)-reducing agent and introduce them into said side stream underconditions effective to gasify said aqueous urea NO_(x)-reducing agent;heating means for maintenance of a temperature of greater than 300° C.to assure substantially all of the urea in said droplets is convertedinto a gas in from 1 to 10 seconds, leaving no significant dissolved orfree solids or liquid to contact with and foul a NO_(x)-reducingcatalyst; means including an ammonia injection grid for introducing saidside stream of gases containing the gases resulting from thegasification of the urea NO_(x)-reducing agent into a primary stream ofNOx-containing gases of greater volume than the side stream to create acombined gas stream.
 2. An apparatus according to claim 1, wherein meansare provided for separating the side stream of gases from a combustiongas stream to produce said side stream of gases and said primary stream.3. An apparatus according to claim 1, wherein means are provided forintroducing outside air as said side stream of gases.
 4. An apparatusaccording to claim 1, wherein means are provided for withdrawing gasesfrom said combined gas stream following their passage through saidNO_(x)-reducing catalyst to form said side stream of gases.
 5. Anapparatus according to any one of claim 1-4, wherein means are providedto heat said side steam of gases to a temperature of at least 200° C.prior to introducing the aqueous solution of urea.
 6. An apparatusaccording to any one of claims 1-4, wherein means are provided tointroduce a urea solution at a rate relative to the NO_(x) concentrationin said combined stream prior to passage through said NO_(x)-reducingcatalyst effective to provide an NSR of from 0.1 to 2.0.
 7. An apparatusaccording to any one of claims 1-4, wherein means are provided tointroduce a urea solution at a concentration of from 5 to 70% by weight.8. An apparatus according to any one of claims 1-4, wherein means areprovided to heat said side stream by the steam to facilitategasification of the NO_(x)-reducing agent.
 9. An apparatus according toany one of claims 1-4, wherein mixing means are provided in said sidestream of gases.
 10. An apparatus for reducing the concentration ofnitrogen oxides in a stream of combustion gases, comprising: conduitmeans for transporting a side stream of gases comprised of outside airand/or combustion gases and comprising less than 3% of the volume of thetotal combustion gases at a temperature of at least 300° C.; meansincluding nozzles to produce droplets of under 500 microns of an aqueoussolution of urea NO_(x)-reducing agent and introduce them into said sidestream under conditions effective to gasify said aqueous ureaNO_(x)-reducing agent; heating means for maintenance of a temperature ofgreater than 300° C. to assure substantially all of the urea in saiddroplets is converted into a gas in from 1 to 10 seconds, leaving nosignificant dissolved or free solids or liquid to contact with and foula NO_(x)-reducing catalyst; means including an ammonia injection gridfor introducing said side stream of gases containing the gases resultingfrom the gasification of the urea NO_(x)-reducing agent into a primarystream of NO_(x)-containing gases of greater volume than the side streamto create a combined gas stream; means for passing the combined gasstream through a NO_(x)-reducing catalyst under conditions effective toreduce the concentration of NO_(x) in the combined gas stream; and aparticle removal means to remove particulates from at least a portion ofthe combustion gases in advance of a NO_(x)-reducing catalyst saidparticulate removal means, and the said side stream is withdrawn fromthe effluent downstream of said particulate removal means.